Charge-transfer-devices have been used in the construction of transversal filters. A charge-transfer-device (CTD) transversal filter includes individual charge-storage cells each connected to separate taps and connected together in series to form a tapped delay line wherein the outputs from each tap are weighted and summed together to produce a filter output. Rather than acting as ideal delay line elements, the individual CTD cells each act as a low-pass filter. That is, the CTD cells are connected together as a delay line in which packets of charge are passed from cell to cell. Each CTD cell has charge transfer inefficiency which expresses the ratio between the amount of charge trapped within the cell, to the amount of charge which was first transferred into the cell. For a completely efficient CTD cell, the amount of charge transferred out would equal the amount of charge transferred in. In practice, CTD cells are less than completely efficient and the charge-transfer inefficiency varies from cell to cell. As a consequence of the way in which the amount of charge in each CTD cell is detected, the sensing of charge levels is different from cell to cell, thus producing what is known as fixed-pattern noise. In the absence of fixed-pattern noise, the sensing of charge would be uniform from cell to cell. The presence of charge-transfer inefficiency (CTI) and fixed-pattern noise (FPN) causes the performance of CTD, transversal filters to differ from the performance which is expected of and computed for, idealized transversal filters.
CTD transversal filters may be used in commercial television sets for ghost cancellation, in aeronautical electronics for multiband-radio communications, and in telephone systems for equalization. CTD filters have particular advantages in allowing the processing of high-frequency signals and allow the production of filters on integrated-circuit micro chips. A typical method for setting the tap weights of a transversal filter is to compute the required tap-weighting coefficients for an idealized transversal filter, given the desired frequency response of the filter, and using standardized computational methods for determining the coefficients. The tap weights used in a transversal filter may be fixed at the time of manufacture of the filter or at some subsequent time by manual adjustment. Also, the tap weights of a transversal filter may be dynamically adjusted through a process of adaptation during the operation of the filter.